MicroRNA (hereinbelow may be referred to as miRNA) is a short non-coding RNA of 21 to 22 bases. A precursor, primary miRNA, is processed by Drosha to produce a pre-miRNA, which is further processed by RISC complex comprising DICER to produce a mature miRNA. It then interferes with mRNA-to-protein translation of numerous target genes that are homologous to itself, suppressively controlling expression of various genes (FIG. 1).
Recently, since the involvement of miRNA in cancer was suggested, intensive studies focused on the relation between cancer and miRNA have been made, beginning to reveal the critical role of miRNA in cancer to control the expression of oncogenes and tumor suppressor genes. It has been shown that expression of many miRNAs is decreased in tumor tissue compared to in normal tissue (see, non-patent references 1 and 2). It is also reported that certain type of miRNA whose expression is decreased in tumor cell functions in a manner like a tumor suppressor gene by suppressing expression of oncogenes (see, non-patent reference 3). On the other hand, there has been reports suggesting, for those miRNAs that are overexpressed in tumor tissue, that oncogenes induce their expression, and that they function in an oncogene-like manner being involved in tumor formation (see, non-patent references 4 and 5).
Besides, studies have also been made to approach to the relation between miRNA and cancer from the aspect that miRNA functions as the target for tumor suppressor genes and oncogenes. For instance, an oncogene product Myc targets miR17-92, a miRNA cluster gene which is highly expressed in B cell lymphoma, and induces its expression (see, non-patent references 4 and 5). On the other hand, a tumor suppressor gene product p53 directly targets miRNA genes of miR-34 family consisting of miR-34a, miR-34b and miR-34c, and it has been shown that p53 is bound to p53-binding site located in the promoter region of miR-34a gene and miR-34b/c gene, thereby controlling the expression of each miRNA of miR-34 family (see, non-patent reference 6). Furthermore, each miRNA of miR-34 has been shown to have an effect of suppressing cell proliferation and inducing G1 arrest (see, non-patent references 6 and 7).
As described above, intensive studies have been made on the involvement of miRNA in cancer, and from their results it has been considered that miRNA analysis in cancer is important in order not only to elucidate the mechanism of carcinogenesis but also to obtain a basic knowledge for developing novel methods for diagnosis and treatment of cancer.
Meanwhile, inactivation of tumor suppressor gene is considered to be one factor of cancer development and/or progression, and a widely known mechanism of it is epigenetic suppression of gene expression (silencing). Particularly, cytosine methylation of a CpG (DNA methylation) in the region of transcription initiation of a gene is a phenomenon observed in almost any cancer. Many of the known tumor suppressor genes are reported to be silenced by DNA methylation. Recent discoveries of a series of genes that are specifically DNA-methylated in cancer imply that DNA methylation in cancer is as important as mutation and deletion as mechanism of genetic abnormalities associated with cancer development and/or progression. In the context of the aforementioned involvement of miRNA in cancer, studies have gradually been made on the relation between the silencing of miRNA by methylation and cancer (see, non-patent references 8 and 9).
Nevertheless, although there are hundreds of miRNAs, only a few among them have been implicated in specific cancer for their DNA methylation, and, many miRNAs are not even described for the specific function of their own. Therefore, most of miRNAs are yet to be analyzed in detail about their involvement in the epigenetic control of gene expression in cancer, and a large part thereof is still unclear.    [Non-patent reference 1] Lu J. et al., Nature, 435: 834-838, 2005    [Non-patent reference 2] Thomson J. M. et al., Genes & Development, 20: 2202-2207, 2006    [Non-patent reference 3] Johnson S. M. et al., Cell, 120: 635-647, 2005    [Non-patent reference 4] He L. et al., Nature, 435: 823-833, 2005    [Non-patent reference 5] O'Donnell K. A. et al., Nature, 435: 839-843, 2005    [Non-patent reference 6] He L. et al., Nature, 447: 1130-1134, 2007    [Non-patent reference 7] Corney D. C. et al., Cancer Research, 67: 8433-8438, 2007    [Non-patent reference 8] Lujambio A. et al., Cancer Research, 67: 1424-1429, 2007    [Non-patent reference 9] Lujambio A. et al., Cell Cycle, 15: 1455-1459, 2007